in THE CHURCH OF SS. SERGIUS AND BACCHUS 77 compartments ; they have no windows., and, what is of special importance to note, they are deeply concave.1 Such marked hollowness is found in later domes as a decorative feature, but here it is primarily and supremely a constructive device. By its means the concave compartments are set slightly back FJG< 2i.~VIEW OF THE EXTERIOR OF THE DOME OF SS. SERGIUS AND BACCHUS. from the octagon's inner face, leaving, at the springing line, portions of the wall-head to appear as little flat ledges on each side of the angles* This is a most skilful expedient, 1 <The centres of the radii of these concave compartments are formed by having three points given the groins on either side and the angle of the octagon in the centre. With these points for each compartment the radius is given, and an arc turned giving the concavity required for each web at its springing.'—A. E. Henderson in the Builder, January 1906, p. 4.